This invention relates to improvements in multi-cylinder radial aircraft engines and more specifically to model aircraft radial engines. Multi-cylinder radial engines are used by scale-model and radio control enthusiasts to provide the utmost in realism and appearance. Because present day radial engines are extremely expensive, most scale modelers use single cylinder engines that are massed produced and low in cost. They then add plastic or non-functional cylinders to simulate the appearance of the radial engine. The single cylinder two cycle engine is overwhelmingly the most common engine used to power engines today, because of its low cost. Consequently, if the price of the radial engine could be substantially reduced, more would be in use.
Present radial model engines are of the four cycle variety. They use the same drive mechanism for producing rotation of the output shaft as their counterpart full size engines. The pistons are equally spaced circumferentially in radial cylinders located on a common crankcase. The piston connecting rods pivot on a common crank plate except for one which is rigidly connected. This rigid joint keeps the plate orientation fixed relative to the crankcase. The center of the crank plate has a bearing that rotates on the crank of the output shaft. Rotation of the output shaft results in nutation of the plate and sequential sinusiodal linear motion of the pistons.
The fact that radial engines have the above mechanism, as well as several cylinders and pistons, obviously increases their cost compared to the same displacement one cylinder engine. Nearly all model radial engines are the four cycle type. They incorporate two valves per cylinder, two push rods per cylinder, and associated cams. All of these parts add to the complexity of the engine and contribute to its high cost. Nevertheless, the four cycle radial engine is used because of its lower noise and reduced shaft speed compared to the two cycle type. Four cycle types do not require a sophisticated muffler as do two cycle counterparts. On existing four cycle radials, each cylinder is exhausted directly to the atmosphere in order to eliminate an expensive muffler system. Four cycle radials are also dominant at present because they operate at higher torque levels and lower speed than would a similar two cycle radial. They can swing a larger propeller to more effectively clear the large diameter of the radial engine.
The high cost of manufacturing a limited number of radial engines is an important problem overcome by the instant invention. The number of modelers who want to purchase scale radial engines, even at a lower price, is not extremely large. So mass production of these engines for cost reduction does not occur. For example, the primary four cycle radial engine manufacturer in the U.S. only produces about 300 five cylinder radial engines per year at a selling price of $1200. In comparison, the mass produced two cycle single cylinder engine of equal power costs about $120. It uses the simple slider crank mechanism with a single rotary valve usually made part of the crankshaft. The piston motion covers and uncovers the intake and exhaust cylinder ports so separate valves are not needed. Parts count is low and so is cost.
A common disadvantage of present art radial engines is that they cannot be perfectly balanced even though the pistons are located in a common plane. A counterweight attempts to offset the weight of the pistons, connecting rods and nutating motion of the crankplate. These forces cannot be compensated for with the addition of weights alone on the crankshaft. A double row radial engine is needed to effect balance but this is not practical for model use. Even a single cylinder slider crank engine cannot be balanced. The crankshaft counterweight can only offset the crank shaking force but not that of the piston.
Vibration of model aircraft engines has long plagued radio control modelers. The airframe of aircraft must be made stronger and heavier to sustain this punishment. Vibration also results in lower reliability of the onboard radio and electronics. Special mounting precautions for receivers, batteries, and servos have to be taken using rubber isolation mounts or packaging them in foam rubber. Consequently, having a perfectly inertia force balanced engine of this invention would be a major advantage.
A third disadvantage of existing multi-cylinder radial engines is their fixed ratio in output speed. The output shaft is directly coupled to the piston motion or speed of the crank. Therefore, the propeller size that can be used with a given engine is fixed within narrow limits.
A fourth disadvantage in present art radial engines results when using the four cycle engine. It produces lower specific power, that is horse power per cubic inch of displacement, compared to the two cycle engine. This has always been a major disadvantage of single cylinder four cycle engines. But recent community concerns over noise are making the four cycle engine more popular. A very quiet 2 cycle radial engine would address this concern.
Having now discussed the main disadvantages of present art radial engines, mainly: high-cost, vibration, inflexible output speed, and lower specific power for the four cycle type; the following objects of the instant invention can be stated. A primary object of the present invention is to configure a radial multi-cylinder engine by grouping together mass produced single cylinder engines and in particular two cycle engines as building blocks. This will permit the use of massed produced individual engines of low cost. Another object of the instant invention is to create a completely forced balanced engine that is free of vibration. This is accomplished by eliminating the prior art crankplate and using geared outputs of the individual engines, each incorporating its own slider crank mechanism. The present art crankplate creates sequential piston motion and does not allow in phase motion of opposed pistons that is required for balancing by the instant invention. Another object of the invention is to increase engine output torque to at least that of the four cycle radial engine by employing gear reduced two cycle engines as components. And yet another object of the invention is to produce different models by changing the gear ratio. This will allow adaptation of the same engine for use with various size propellers. And another object of the present invention is to integrate in a compact manner an exhaust manifold and muffler to produce a realistic sound at a low noise level for muffling two cycle engines. Another object of the invention is to provide a single throttle means that controls the individual carburetors of each building block engine. Yet another object of the invention is to create a realistic, aesthetic looking engine where the front view of the cylinders is unobstructed by the gearing means, throttle means, or carburetors, to mimic the appearance of full size engines. And yet another object of this instant invention is to create an engine of high specific power by utilizing two cycle high speed high power engines as building blocks that are readily available and mass-produced inexpensively. This does not preclude the use of four cycle engine building blocks. A final object of the instant invention is to provide counter-clockwise propeller rotation, as this is standard.
These and other objects of the present invention are accomplished in accordance with a preferred embodiment of the present invention. For illustrative purposes only, 6 model aircraft engines are equally spaced and radially oriented by mounting them to a common circular plate using their existing rear crank case cover screws. This eliminates the need for a separate mount while allowing each engine to be rearward facing. A second plate is rigidly mounted to the first using spacers and creates the engine structural frame. Holes in the rear plate also serve to mount the radial engine to a fire wall. The end plates centrally house ball bearings that support the output propeller shaft.
The shaft of each building block engine has a pinion gear mounted there on. These gears mesh with a central or sun gear and transmit their power to the output shaft. Outboard ball bearings are positioned in the second plate to support the end of each engine's shaft. This is advantageous so that low cost engines can be used that do not employ crank shaft ball bearings and are not designed for supporting over hung loads. The outboard ball bearings also locate the shaft at an accurate center distance for proper gear mesh, eliminating the need for adjustments.
By using an even number of radial engines located in a single plane, pairs of pistons result that are diametrally opposite or 180 degrees apart. They are timed to fire at the same time, unlike existing radial engines that must sequentially fire. Any pair of opposed pistons moves radially outward or inward in synchronism, thereby inertia balancing one another. Since the crank mechanisms are balanced with counterweights and are identical, perfect balance of paired engines results. Consequently, the radial engine as a whole is completely force balanced.
The output gear ratio can be readily changed even though the center distance is fixed for a given engine. Using smaller diameter pinion gears and a larger output gear will reduce the output speed to accomodate a larger propeller. Thus different models are readily produced of the same basic engine to meet the needs of the customer.
The use of rearward facing individual engines positions the gear train and carburetor to the back of the engine where they may be readily hidden, if desired, inside the cowel of the model aircraft. When viewed from the front, unobstructed radial cylinders are seen. The normal counter clockwise rotation of each building block engine is now clockwise when viewed from the front of the radial engine. The use of gearing reverses this and again results in counter clockwise rotation of the output shaft. Counter clockwise propeller rotation is highly desireable as this is the normal or accepted direction for propeller rotation. Even though the use of gearing is preferred, a belt drive is not precluded.
Each building block engine utilizes its own carburetor for mixture control. These carburetors are mass produced and are supplied with each engine. A simple linear motion linkage, synchronizes and simultaneously operates the individual carburetor air intakes. The linkage is located at the rear of the engine. It consists of a spider to hold 6 radial control wires that attach to the carburetors. The spider slides on a pin located on the centerline of the engine to actuate the carburetors. This symmetry results in equal activation. In an alternate embodiment, a single carburetor can be used in conjunction with an intake manifold that supplies each engine's inlet, but this does not perform as well because of its excessive dead volume.
A round doughnut shaped muffler is located at the front of the engine. It is screwed to the front plate using thermally insulated spacers. Each radial cylinder exhausts into an exhaust tube that is attached to the cylinder exhaust port using existing holes. The tube diverts the cylinder exhaust into the muffler which also serves as an exhaust manifold. Using equally spaced radially entering exhaust tubes has been found to result in very low noise levels. The muffler is donut shaped to allow passage of the propeller shaft through its center. The exhaust exits thru preferably a single port directing fumes and oil away from the engine. When viewed from the front, the muffler simulates the crank case of real aircraft engines. The muffler is synergistically used for noise abatement, as an exhaust manifold, and as a visually appearing crank case. Being located at the front, direct air flow from the proppeller keeps it cool and allows effective cooling of the muffler gasses.